The widespread use of motor vehicles unavoidably results in large quantities of used tires. Globally, it is estimated that about 1.5 billion waste tires are produced every year. In the past, used tires were mostly disposed in landfills, which is not a sustainable solution. As more and more discoveries find that discarded tires pose serious environmental and health threats to our society, proper recycling of worn-out tires has become a critical issue. More recently other usages of ground rubber tires have been found. In 2003 nearly 290 million scrap tires were generated in the United States, and almost 80% of those waste tires were consumed in applications for fuel, as additives in civil engineering applications, and other uses. The recycled tires are mainly consumed as fuel, additives to plastics, rubbers, or civil engineering applications.
The tire rubber formulation contains significant quantities of carbon black that is used as reinforcing fillers and abrasive resistance for rubber matrices. Typically, a tire consists of natural rubber, synthetic polyisoprene, butadiene rubber, styrene-butadiene rubber, carbon black and a fractional amount of additives. High structure carbon black made of clusters of ˜10-100 nm size fundamental particles are used in tire rubber formulations to enhance mechanical properties of the product. The regular direct pyrolysis process results in the production of about 30-40 wt. % carbon black, depending on the pyrolysis conditions. Rubber particles do not exist as a single fundamental particle; rather they are fused together during production of black to make aggregates of various structures. Such structures are retained in vulcanized rubber products such as pneumatic tires that contain dispersed phases of carbon black in rubber matrix.
The waste tire rubber is usually cryogenically pulverized into small micron-sized rubber particles. Cut rubber pieces are also ground in ambient conditions to get powder buffing. Those powdered tire rubbers are usually used as fillers in various low-cost rubber or plastic products. Isolation of the carbon black from tire formulations was tried but such products are not necessarily good reinforcing fillers for a new rubber formulation. Utilization of tire rubber materials for value-added applications would be very attractive not only for the recovery of materials but also to control global pollution.
The search for suitable electrode materials for sodium-ion batteries has become more urgent owing to the great need for large-scale energy storage. With the concerns of the limited global availability of lithium resources and high cost, sodium-ion batteries (SIBs) are considered to be an alternative to lithium-ion batteries (LIBs) for stationary grid energy storage of electricity produced from renewable sources. Due to its high abundance, low cost, and suitable working chemical potential (−2.7 V vs. Standard Hydrogen Electrode), rechargeable sodium-ion batteries are gradually attracting a lot of attention. Sodium shares many similar chemical properties with lithium because of its location in the periodic table and the similarities of fundamental principles of SIBs and LIBs. Thus far, several suitable cathode materials have been developed for SIBs. However, the absence of good anode material hinders the application of SIBs. Unlike the successful application of graphite as anodes in LIBs, the electrochemical sodium insertion into graphite is proven to be not favorable. Theoretical calculations suggest that the interlayer distance of graphite is too small to accommodate the large Na+ ion, and a minimum interlayer distance of 0.37 nm is believed to be good for Na+ insertion. In this regard, a variety of carbon materials have been investigated as anodes for SIBs, such as hard carbons, carbon nanotubes, reduced graphene oxides, and expanded graphite. Hard carbon is likely to be the most promising because of its stability, high capacity, and easy scale-up. However, great amounts of the currently studied hard carbon are produced from sucrose, banana peels and dopamine among others. Most of the precursors are relatively high cost or require complex treatments, which prevent the application in many cost sensitive fields. A low-cost hard carbon anode material is desired for promoting the development of SIBs for large-scale energy storage market.